The present invention relates to an ultraviolet-screening scale (flake) pigment comprising scale-like (flaky) inorganic particles having their surfaces coated with a zinc oxide, a process for preparing such pigment, and cosmetics containing such pigment.
The ultraviolet rays existing in nature are roughly divided into two types according to wavelength. One is called UV-A having wavelengths ranging from 320 to 400 nm and the other is UV-B having wavelengths less than 320 nm. It is said that UV-A, when applied to the human skin, causes pigmentation, while UV-B is causative of suntan (see, for example, "Shikizai (Toner)", 63 (3), 171-175, 1990).
The pigments used for the anti-suntan cosmetics are required to have an ability to effectively screen the ultraviolet rays. Many studies have been made on this subject, and various ultraviolet-screening pigments have been proposed.
For instance, Japanese Patent Application Laid-Open (Kokai) No. 49-450 (1974) proposes an ultraviolet-protecting cosmetic composition containing fine titanium dioxide particles.
Also, Japanese Patent Application Laid-Open (Kokai) No. 62-228006 (1987) proposes fine zinc oxide particles as a substance which can effectively screen UV-A having wavelengths around 360 nm.
Further, Japanese Patent Application Laid-Open (Kokai) No. 61-257909 (1986) proposes cosmetics containing particles having a specific surface area of 15 to 100 m.sup.2 /g and coated with zinc white and/or zinc carbonate, and Japanese Patent Application Laid-Open (Kokai) No. 62-16408 (1987) discloses hiding cosmetics blended with mica particles having their entire surfaces coated uniformly with one or more of metallic oxides or metallic hydroxides such as oxides or hydroxides of titanium, zinc or ferrite to a thickness of 5 to 30 nm.
Fine titanium dioxide particles have an ultraviolet screening effect, but their screening effect is low against the ultraviolet rays with wavelengths of not less than 350 nm which falls within the wavelength region of UV-A.
Also, fine zinc oxide particles, which have been reported to be capable of effectively screening UV-A with wavelengths around 360 nm, are poor in dispersibility when blended in cosmetics and also costly.
Further, in the case of the inorganic particles coated with a zinc-based substance, disclosed in Japanese Patent Application Laid-Open (Kokai) No. 61-257909 (1986), since the zinc-based substance is merely deposited on the surfaces of the inorganic particles by dry-mixing, the substance is not uniformly coated on the surfaces of the inorganic particles, therefore the screening effect thereof against UV-A is not satisfactory.
In the case of mica coated with an oxide of a metal such as zinc, disclosed in Japanese Patent Application Laid-Open (Kokai) No. 62-16408 (1987), the object thereof is to improve the surface properties of mica, and the coating amount of the metallic oxide is too small to provide a sufficient screening effect against UV-A.
As viewed above, any of the hitherto proposed ultraviolet-screening pigments had some disadvantages such as unsatisfactory screening effect against ultraviolet rays, poor dispersibility, etc.
Thus, an offer of a pigment for cosmetics which has an excellent ultraviolet-screening effect, especially high screening effect against UV-A and a good dispersibility in cosmetic compositions, is demanded.
As a result of the present inventors' studies, it has been found that by suspending scale-like inorganic particles in water, adding an aqueous solution of a water-soluble zinc compound into the suspension, adjusting the pH of the resulting suspension to 9 to 11, hydrolyzing the zinc compound while gradually lowering the pH of the suspension to 5.5 to 7.5 over a period of one to 10 hours, and baking the resulting scale-like inorganic particles coated with the zinc compound hydrolyzate at a temperature of 300.degree. to 700.degree. C., the obtained scale pigment coated with a zinc oxide has a high screening effect against ultraviolet rays including UV-A, shows good dispersibility in cosmetics when blended therein, and is capable of providing cosmetics which are excellent in tactile properties. The present invention has been achieved on the basis of this finding.